Typically, iron ore is converted to steel through basic processes that have been known for many years. These processes usually involve the conversion of iron ore to pig iron in a blast furnace using coke produced in a coke oven, and the subsequent conversion of the pig iron, or hot metal, to steel in an open hearth or basic oxygen furnace. However, the high energy and capital costs involved with making steel in the traditional manner have created a demand for new, less expensive methods for producing steel. More specifically, a great deal of effort has been directed to the elimination of the blast furnace and coke oven in steel-making. Blast furnaces use large quantities of energy, the cost and availability of which is becoming more and more uncertain. Additionally, coke ovens are a large source of pollutants, and modifications to existing coke ovens to meet government regulations are becoming prohibitively expensive.
Accordingly, some effort has been directed to the conversion of iron ore directly to iron carbide followed by the production of steel from the iron carbide, thereby eliminating the blast furnace step.
In this regard, U.S. Reissue Pat. No. Re 32,247 by Stephens, Jr. discloses a process for the direct production of steel. Iron oxides in iron ore are converted to iron carbide, and steel is then produced directly from the iron carbide in a basic oxygen furnace or electric furnace. The electric furnace is typically an electric arc furnace, although it is possible to use other electric furnaces, such as an induction furnace. In the direct production process, the iron oxides in the iron ore are reduced and carburized in a single operation using a mixture of hydrogen (as a reducing agent) and carbon bearing substances (as carburizing agents). The process is typically carried out in a fluidized bed reactor. Steel is then produced by introducing the iron carbide into a basic oxygen furnace or electric furnace, with the blast furnace step being eliminated.
While the process of Stephens, Jr. has proven to be an important advance in the art, a need exists for further improvements in this method of directly producing steel. For example, in the step of converting the iron oxides into iron carbide, even minor variations in the process parameters can cause inferior results, e.g., minor variations in the interrelated process parameters of temperature, pressure and gas composition can cause free iron (Fe) or a variety of iron oxides such as Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, and FeO to be produced, rather than iron carbide.
The usefulness of preheating iron ores prior to being placed in a fluidized reactor bed is described in the prior art. However, none of these prior art references disclose the preheating of iron oxides in conjunction with a process of the type disclosed by Stephens, Jr.
U.S. Pat. No. 2,752,234 by Shipley discloses a process for the direct reduction of iron ore to iron. The raw iron ore is preferably preheated to a temperature of about 700.degree. F. (370.degree. C.) by off-gases from the reaction comprising hydrogen and carbon monoxide.
U.S. Pat. No. 2,864,688 by Reed discloses a method of forming iron including the step of preheating iron oxide-containing raw material that is a mixture of hematite and magnetite, to a temperature of about 1700.degree. F. (930.degree. C.).
Other patents that disclose the preheating of iron ore prior to being placed in a reactor bed include U.S. Pat. No. 2,894,831 by Old et al., U.S. Pat. No. 2,921,848 by Agarwal, U.S. Pat. No. 3,021,208 by Feinman, U.S. Pat. No. 3,761,244 by Hoffert, U.S. Pat. No. 3,928,021 by Matsubora et al., U.S. Pat. No. 4,045,214 by Wetzel et al., U.S. Pat. No. 4,360,378 by Lindstrom, U.S. Pat. No. 4,202,534 by Davis Jr. and U.S. Pat. No. 4,851,040 by Hoster et al.
However, none of the prior art references describe a process for preheating iron oxide ore that would be beneficial to an iron carbide production process. The chemistry and equilibrium conditions for forming iron carbide are significantly different than the conditions necessary for forming metallic iron.